Detecting and evaluating optical combustion chamber signals with the aid of optical transducers is known (MZT 40 (1979), page 242). In this case, the measurement criterion is the presence or absence of light in the combustion chamber as detected by threshold detectors. Controlling the instant of ignition in internal combustion engines with the aid of an optical transducer is also known (German Offenlegungsschrift No. 25 20 325). In the known apparatus, the optical transducer may be a phototransistor, by way of example, which is followed by a threshold switch which detects a voltage increase at the transducer output. This voltage increase is a criterion for the initiation of the second phase of combustion in the combustion chamber of an internal combustion engine. The second phase of combustion follows a first phase, in which the mixture is ignited but in which there is no significant pressure increase and no significant emission of light. In contrast, the second phase is marked not only by a well-defined pressure and temperature increase, but also by the emission of light. It is furthermore known to evaluate the course of pressure, detected with suitable pressure sensors for open- and closed-loop control purposes.